CLINOHEDRITE Chapter15. Nesosilicates

CLINOHEDRITE

CaZn(SiO₄).H₂O
Monoclinic, Cc, a = 5.16, b = 15.94, c = 5.41 Å b = 103.90^o, Z = 4.




	Figure 15-35. Crystal drawings of clinohedrite from Franklin; the two at the top are from the Parker Shaft area. Drawings are from Palache (1935) who provided crystallographic data.

Clinohedrite is one of Franklin’s more remarkable minerals, but is not found at Sterling Hill. It was first described by Penfield and Foote (1898, 1901) from the Parker Mine, and much information was added by Palache (1935). Unit-cell parameters were first determined by Strunz (1941). X-ray powder data are given by the ICDD (PDF# 17-214).

Crystal structure

The crystal structure of clinohedrite has been the subject of a number of investigations; these were by Nitikin and Belov (1966), Venetopoulos and Rentzeperis (1976), and Simonov et al. (1977a). Simonov et al. (1978) also pointed out a relationship to gerstmannite. Venetopoulis and Rentzeperis (1976) described clinohedrite as having (ZnSiO₄) layers parallel to (010) and interlayered with layers of Ca octahedra, in which Ca is coordinated by 4 oxygen atoms and two water molecules.

Description




	Figure 15-36. Crystal drawings of clinohedrite from the Parker Shaft area in Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Clinohedrite occurs in superb crystals up to 1 cm, but most are smaller, occurring in millimeters. It also occurs as granular, fibrous, and platy aggregates and as splayed plumose arrays in veins.

The crystals, of extremely complex and hemimorphic morphology, have been studied by Penfield and Foote (1898, 1901), and the morphological data was expanded and summarized in great detail by Palache (1935), who also presented numerous crystal drawings (Figures 15-35 and 15-36). Other crystals, showing some of clinohedrite’s great morphological diversity, are illustrated in figures 15-37 through 15-39. Clinohedrite crystals are predominantly pinacoidal, pseudoprismatic, and domatic in habit. Many crystals appear wedge-shaped, having inclined faces; this characteristic prompted the name of the species. Additionally, crystal faces are commonly curved. The color varies from colorless to white to pink to light violet. The luster is vitreous on crystal faces and fracture surfaces and pearly on the perfect {010} cleavage. The density is 3.33 g/cm³.

Optically, clinohedrite is biaxial, negative, with a = 1.662, b = 1.667, and g = 1.669. The fluorescence in ultraviolet is orange in both shortwave and longwave, but is not wholly consistent; some rare specimens do not exhibit orange fluorescence. In general, shortwave fluorescence is more intense than longwave, but varies in intensity from assemblage to assemblage; some violetish specimens have a weaker, pinkish-orange fluorescence (some observations courtesy of Chet Lemanski). The duration of phosphorescence was measured by Millson and Millson (1950) and was found to be 175-288 hours. Clinohedrite is also strongly pyroelectric and triboluminescent.

It is commonly distinguished from other species using ultraviolet fluorescence, although wollastonite, pectolite, johnbaumite, and other minerals have a similar fluorescent response; specimens which are apparently non-fluorescent are best verified using optical, chemical, or X-ray methods.

Composition




	Table 3. Chemical analyses of hardystonite, hodgkinsonite, gerstmannite, clinohedrite, gageite, and yeatmanite.

Clinohedrite is a calcium zinc silicate hydrate mineral. Several modern analyses are given in Table 3. These and others not provided here show that this mineral conforms to the end-member composition, with little substitution of other cations.

Occurrence and paragenesis

Clinohedrite occurs in two principal types of assemblages at Franklin, although there are many exceptions; it is always a secondary mineral. It commonly occurs as thin films and coatings on and near hardystonite, Ca₂ZnSi₂O₇, from which it has formed by direct hydrothermal alteration. It may form complete replacements of hardystonite.




	Figure 15-37. Clinohedrite crystals from Franklin; this is the typical habit. Field of view is 2.7 mm in maximum dimension.		Figure 15-38. Clinohedrite in a rare habit showing flaky composite crystals from Franklin. Field of view is 1.2 mm in maximum dimension.




	Figure 15-39. Clinohedrite crystals in triangular, tabular, uncommon habit, from Franklin. Field of view is 1.8 mm in maximum dimension.

Clinohedrite also occurs as aggregates, crystals, druses of crystals, and vug-and-crack fillings in many late-stage recrystallized assemblages, most notably with hancockite, andradite, ganophyllite (Figure 18-35), hodgkinsonite, manganaxinite, and many lead silicates. One well-documented sequence of a complex assemblage was published by Dunn et al. (1983a). The known assemblages are numerous; some of these are:

(1) Clinohedrite occurs as a late-stage, recrystallized, colorless to faint pink to faint violet mineral associated with a large number of species encountered from the Parker Mine at the end of the last century. This is the type assemblage (Figure 15-37) in which it occurs with manganaxinite, franklinite, willemite, andradite, roeblingite, nasonite, xonotlite, ganomalite, charlesite, datolite, hancockite, and a host of other species, in vein and solution cavities. Hardystonite is moderately rare or absent in this assemblage.

(2) Clinohedrite occurs as masses which have almost completely replaced hardystonite and serve as a pseudomorphic host for glaucochroite crystals, associated with andradite and cuspidine, as described by Leavens et al. (1987). Specimens are rare.

(3) Clinohedrite is associated with larsenite and hodgkinsonite as direct hydrothermal alterations of a hardystonite-esperite-glaucochroite matrix (clinohedrite forming from hardystonite, hodgkinsonite forming from glaucochroite and willemite, and larsenite forming from esperite). Specimens are rare.

(4) Clinohedrite with willemite, hardystonite, and massive light brown glaucochroite in 5 cm thick veins in franklinite.

Additionally, clinohedrite occurs with a number of rare species, such as gageite, cahnite, and others, in many one-of-a-kind assemblages. As a hydrothermal mineral, it can be found with many assemblages, albeit in minor amounts.

Name

Clinohedrite is named for the Greek words for incline and face, in allusion to the morphology induced by domatic forms.

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